Conventionally, an X-ray fluoroscopic imaging technique has been studied, which obtains a contrast image by utilizing the difference of X-ray absorption abilities. However, as an element becomes lighter, the X-ray absorption ability becomes smaller, resulting in a problem that enough contrast cannot be expected for biological soft tissue and soft material. In view of above mentioned problem, an imaging method which generates the contrast based on a phase shift of the X-ray has been studied in recent years. An imaging method using the Talbot interference is one of X-ray phase imaging methods utilizing such a phase contrast.
FIG. 8 illustrates an exemplary configuration of the Talbot interference method. The imaging by the Talbot interference method needs at least an X-ray source 19 which can be spatially interfered, a phase-type diffraction grating 21 (hereinafter, described as phase grating) for cyclically modulating a phase of the X-ray, and a detector 23. An X-ray intensity distribution of the X-ray which can be spatially interfered, after it transmits through the phase grating 21, resembles the shape of the phase grating 21. When a spatial coherence length of the X-ray is larger than a pitch of the phase grating 21, a light-dark cyclic image with high contrast appears at a location of (d2/λ)×a/8. Here, “d” is a pitch of the phase grating 21, “λ” is a wavelength of the X-ray, and “a” is an odd integer. In the present specification, the pitch of the phase grating means a cycle in which the grating is arranged. The pitch of the phase grating may be a distance C between centers of one grating and other grating neighboring to the grating, or a distance C′ between sides of two consecutive gratings, as illustrated in the schematic diagram of the phase grating shown in FIG. 9.
As described above, a Talbot effect is a phenomenon in which a light-dark cyclic image is cyclically formed at a specific distance between the phase grating 21 and the detector 23. This light-dark cyclic image is referred to as a self-image. When a test object 20 is located in front of the phase grating 21, the irradiated X-ray is refracted by the test object 20. Thus, a phase image of the test object 20 can be obtained by the detecting of the self-image which is formed by the X-ray refracted because of transmitting the test object 20. However, an X-ray image detector with high spatial resolution becomes necessary to detect the self-image generated with enough contrast. In such a case, an absorption grating 22 can be utilized, which is made of material absorbing the X-ray, and having a sufficient thickness. When absorption grating 22 is located in a location in which the self-image is formed, moiré fringes are generated by the overlapping of this self-image and the absorption grating 22. That is, information on the phase shift can be observed by the detector 23 as modification of the moiré fringes.
By the way, when it is necessary to observe in a high resolution, it is more desirable that the pitch of the phase grating 21 is smaller. On the other hand, as the phase grating, some thickness (height) becomes necessary to π-shift the phase of the X-ray. Meanwhile, as illustrated in a schematic diagram of FIG. 9, in the present specifications, the thickness (height) of the phase grating means the thickness (height) of a projection part which is a long side of the grating indicated by “B”. A width of the projection part means a width indicated by “A” in the above FIG. 9. An aperture width of an aperture part means a distance between the projection parts, which is indicated by “A′” in the above FIG. 9.
“Aperture width” and “width of the projection part” of the phase grating are generally formed by 1:1. Here, when the pitch of the phase grating 21 is caused to be smaller to make the phase grating 21 with the high resolution, it is also necessary to downsize the width of the projection part and the aperture width. Thus, such a problem is induced that an aspect ratio becomes larger, which is defined by the thickness (height) of the projection part/the aperture width of the aperture part, or the thickness (height) of the projection part/the width of the projection part, so that it becomes difficult to make the phase grating 21. For example, when Si is used as material of the phase grating 21, the thickness necessary to π-shift the phase of the X-ray of 20 keV is approximately 29.2 μm. When the slit-like phase grating 21 is made, it is required to make the slit-like phase grating 21 with a pitch of approximately 2 μm, that is, the aperture width of 1 μm because of the desired resolution. In this case, the aspect ratio becomes approximately 30, resulting in that it is difficult to make the diffraction grating with a large area. Thus, in US Patent Application Publication No. 2007/0183579 (now U.S. Pat. No.: 7,639,786) specifications, a partial grating with the low aspect ratio is used to make the diffraction grating whose apparent aspect ratio is high. Specifically, as illustrated in FIG. 10, partial gratings 30, which can be easily made, with the lower aspect ratio are stacked in a virtual direction, thereby, the diffraction grating, whose apparent aspect ratio is high, is made.